CSS Drive – CSS Compressor – Use this utility to compress your CSS to increase loading speed and save on bandwidth as well. You can choose from three levels of compression, depending on how legible you want the compressed CSS to be versus degree of compression. http://www.cssdrive.com/index.php/main/csscompressor

Do I need to decrease my css file size?
I guess the quick answer is yes, file size affects users loading times which will help users have better experience on your website and reduce their bandwidth usage especially if they are loading your website on a handheld device.